1. Technical Field
This invention relates to the transmission of data between objects which are moving relative to one another, e.g. a moving transmitter and a stationary receiver. It is particularly, but not exclusively, applicable to acoustic data communications at ultrasound frequencies (of the order of 40 kHz) in air.
2. Background Information
PCT Patent Application Publication No. WO 03/087871 discloses a locating system based on ultrasonic communications which is able to determine in which room each of a number of ultrasonic transmitter tags is located. Each tag transmits a unique identification signal which is picked up by a one of the receivers which are provided in every room
The Applicant has observed that current ultrasonic positioning systems such as this use an acoustic data link which is very restricted in its data rate. This limits the number of objects/persons that can be tracked and/or the update rate and in particular how well rapid movements of many persons/objects in and out of rooms can be followed with accuracy.
The most advanced acoustic communications systems of which the applicants are aware are those that are found in underwater acoustics. The first generation of digital modems were based on frequency shift keying (FSK), as FSK is robust in terms of time and frequency spreading of the channel. But FSK is inefficient in how it uses bandwidth, so in recent years there has been a large effort in developing more efficient coherent systems based on e.g. various forms of phase shift keying (PSK), as described for example in the article by D. B. Kilfoyle and A. Baggeroer, The state of the art in underwater acoustic telemetry, IEEE Trans. Ocean. Eng., OE-25, 1-1111 (2000), often in combination with adaptive equalization. Despite this, incoherent FSK and MFSK (multiple FSK) systems play a large role in providing reliable communications in practice. Such systems are typically non-adaptive and designed with sufficient bandwidth to accommodate the harshest environment expected. This means that under ordinary, more favourable conditions the systems will be operating inefficiently with respect to bandwidth and power. Such inefficiencies can be substantial. One of the design constraints that causes this low bandwidth efficiency is the presence of frequency shifts due to the Doppler effect.
Wherever a transmitter and receiver are moving towards or away from each other the frequency of the signal perceived at the receiver differs from that transmitted by the transmitter as a result of the differing distance that each wavefront must travel between the two. This is known as the Doppler effect.
The relatively low value of the speed of sound causes even low speed movements to create relatively large frequency shifts. A relative movement of v, where a positive v means movement from the source towards the receiver, shifts the frequency to:f′=f(1+v/c)  (1)Where f is the original frequency and c is the velocity of sound (e.g. about 340 m/s in air and about 1500 m/s in water).
As an example, an underwater acoustic communications system operating at a centre frequency of 25 kHz and which is used on an AUV (Autonomous Underwater Vehicle) with a velocity of 10 knots will be Doppler shifted by 86 Hz or 3.4% of a typical relative bandwidth of 10% of the centre frequency (i.e. 2500 Hz). An airborne ultrasound communications system transmitting at 40 kHz from a transmitter which is moving at a speed of 6 km/h (fast walking) will experience an even larger Doppler shift of 196 Hz or 4.9% of the typical relative bandwidth of 10% (i.e. 4000 Hz).
The Doppler shift will generate a shift up or down in frequency depending on the relative motion. MFSK uses multiple frequencies simultaneously and can be considered to be several FSK systems working in parallel. The only relationship between the frequencies is that they should not be allowed to overlap. In an MFSK system, it is theoretically possible and desirable to space frequencies as close as the inverse pulse length, B=1/T. However, the Doppler shift, fD=f′−f may easily exceed this spacing by a large amount, |fD|>>B, and thus effectively limit the number of frequencies that can be used and consequently also the bit rate.
The standard way to accommodate Doppler shifts is to space frequencies according to the maximum Doppler shift plus a certain guard band, fg:Δf>B+2|fd|+fg  (2)The ratio of Δf and B can be substantial. As transducers have a limited bandwidth this represents a loss in the effective data rate which can be achieved in accordance with this scheme.
The modulation schemes described so far are adaptations of methods that work well in radio communications. It is however an object of the invention to provide a scheme more appropriate for acoustic environments.